It's pretty decent, it's basically regular Photoshop but missing some bells and whistles (mostly the really advanced colour adjustments that are far too complicated for me to understand). It can be pretty basic but gets the job done. What's more, the version I've been using (3.0) is ridiculously out of date at the moment (I think they've just released version 6.0).
I recommend sorting out a tablet, though, if you're thinking of using something like Photoshop for your art. It just frees you up to be much more spontaneous with your art and colour. I got one back in February and there has been (I think) a very noticeable improvement in my work since then. It's not an essential tool, by any means, I personally felt I needed one because I was having difficulty (as a left-handy) using a right-handed mouse to colour and fix my line art.